Photochromic compounds typified by chromene compounds, fulgide compounds and spirooxazine compounds have a characteristic feature (photochromic properties) that they change their colors swiftly upon exposure to light including ultraviolet light such as sunlight or light from a mercury lamp and return to their original colors when they are put in the dark by stopping their exposure to light and are used for various purposes, especially optical materials, making use of this characteristic feature.
For example, photochromic spectacle lenses which are provided with photochromic properties by using a photochromic compound function as sunglasses which are quickly colored outdoors where they are irradiated with light including ultraviolet light such as sunlight and as ordinary transparent eyeglasses which are faded indoors where there is no irradiation, and demand for the photochromic spectacle lenses is growing nowadays.
To provide photochromic properties to an optical material, a photochromic compound is generally used in combination with a plastic material. Stated more specifically, the following means are known.
(a) A method in which a photochromic compound is dissolved in a polymerizable monomer and the obtained solution is polymerized to directly mold an optical material such as a lens.
This method is called “kneading method”.
(b) A method in which a resin layer containing a photochromic compound dispersed therein is formed on the surface of a plastic molded article such as a lens by coating or cast polymerization.
This method is called “lamination method”.
(c) A method in which two optical sheets are bonded together by means of an adhesive layer formed of an adhesive resin containing a photochromic compound dispersed therein.
This method is called “binder method”.
For optical materials such as optical articles provided with photochromic properties, the following properties are further required.
(I) The degree of coloration at a visible light range before ultraviolet light is applied (to be referred to as “initial coloration” hereinafter) should be low.
(II) The degree of coloration upon exposure to ultraviolet light (to be referred to as “color optical density” hereinafter) should be high.
(III) The speed from the stoppage of the application of ultraviolet light to the time when the compound returns to its original state (to be referred to as “fading speed” hereinafter) should be high.
(IV) The repeat durability of a reversible function between color development and fading should be high.
(V) Storage stability should be high.
(VI) The compound should be easily molded into various shapes.
(VII) Photochromic properties should be provided without the degradation of mechanical strength.
When optical materials having photochromic properties are to be produced by the above means (a) to (c), various proposals have been made to satisfy the above requirements. As for color optical density and fading speed, it is now desired that more excellent photochromic properties should be developed.
For example, since the above-described kneading method has an advantage that photochromic plastic lenses can be mass-produced at a low cost by using glass molds, most of photochromic plastic lenses are now manufactured by this method.
However, as strength is required for a lens substrate in the kneading method, it is necessary to enhance the mechanical strength of a matrix resin containing a photochromic compound dispersed therein. Therefore, it is difficult to develop excellent photochromic properties. That is, since the degree of freedom of molecules of the photochromic compound existent in the matrix resin becomes low, a photochromic reversible reaction is impaired.
For example, as for this kneading method, Patent Document 1 discloses a technique for adding a photochromic compound to a monomer composition comprising an isocyanate monomer and a thiol monomer. Patent Document 2 discloses a photochromic curable composition comprising a specific (meth)acrylic polymerizable monomer and a photochromic compound.
Therefore, photochromic lenses molded by polymerization-curing these compositions are unsatisfactory in terms of photochromic properties, especially fading speed, though they have high mechanical strength.
Meanwhile, in the lamination method and the binder method as compared with the above-described kneading method, since photochromic properties are developed with a thin layer formed on the surface of a substrate, to develop the same color optical density as that of the kneading method, a photochromic compound must be dissolved in a high concentration. In this case, there occurs a problem such as unsatisfactory solubility or precipitation during storage according to the type of a photochromic compound. Further, since the layer which develops photochromic properties is thin, the photochromic compound may be inferior in durability.
For example, Patent Document 3 discloses that a photochromic curable composition is applied to a plastic lens by spin coating and optically cured to form a photochromic coating layer (this lamination method is also called “coating method”).
Patent Document 4 discloses a photochromic layer method (to be also referred to as “two-stage polymerization method” hereinafter) in which a photochromic layer is formed by securing a space between a plastic lens and a glass mold by means of a member such as an elastomer gasket, adhesive tape or spacer, pouring a photochromic curable composition into this space and polymerization-curing the composition.
Further, Patent Document 5 discloses that a laminated sheet is produced by bonding together transparent carbonate sheets by means of a polyurethane resin adhesive layer containing a photochromic compound (binder method).
Therefore, since photochromic properties are developed with a thin layer containing a photochromic compound in all of Patent Documents 3 to 5, when a photochromic compound having low solubility is used, color optical density tends to become low and further, the photochromic compound may be inferior in durability.
Thus, according to the current known technologies, either one of color optical density and fading speed tends to become unsatisfactory.